Fluorescent linear lamps generally have a circular cross section and have an emitted light output from their glass enclosure along their 360 degree radius about their longitudinal axis. They therefore provide light that radiates equally outwardly from their glass enclosure, which means that some of the light radiates directly out of open fluorescent fixtures in which they are mounted and some light is directed back into the fixture. Thus, when fluorescent linear lamps are installed in lighting fixtures, for example lighting fixtures that are suspended from or below a ceiling, the illumination is projected onto the ceiling in addition to downward toward the floor. In order to redirect some of this upwardly directed light downwardly where it is needed, most fluorescent linear lamp fixtures have, for example, a shiny mirrored or glossy white painted reflectors positioned above the fluorescent linear lamp. In many retail and commercial offices and warehouses that use high bay fluorescent fixtures or suspended fixtures, these fixtures have slots in the metal which allows a percentage of the illumination to exit the top of the fixture and illuminate the ceiling. Typically about 10% to 15% of the light is provided to up light.
While use of reflectors does help reflect some of the light back out, still some of the light is nonetheless absorbed and fails to be directed where it is needed for efficient illumination. This results in some of the light being wasted, which also represents a waste of electricity.
In contrast with fluorescent linear lamps, LED linear lamps typically provide illumination within about a 180 degree range of transmission. This is due to the fact that the individual light emitting diodes (LEDs) in the LED linear lamps are surface mounted on flat circuit boards located inside the center of a transparent or translucent tube. In some embodiments the flat circuit board is positioned near a center of the transparent or translucent tube, for example, as shown in FIG. 5, and light will exit the lower half of the transparent or translucent tube. In some other embodiments the flat circuit board is positioned nearer to one side of the transparent or translucent tube, for example as shown in FIG. 4, and light will exit through more than 180 degrees along the transparent or translucent walls of the transparent or translucent tube (in this embodiment about a 270 degree range.) Nonetheless, since the LEDs remain surface mounted to a flat circuit board positioned in the transparent or translucent tube, the light will still only project out along a 180 degree arc.
When a LED linear lamp is placed within a lighting fixture mounted on a ceiling with the LEDs directly downwardly, the light from the LEDs is projected downwardly toward the floor in a main illumination direction. Conversely, when a LED linear lamp is placed within a lighting fixture mounted below a ceiling with the LEDs directly upwardly to reflect on the ceiling, the emitted light is projected upward toward the ceiling. LED linear lamps do not have the capacity to simultaneously project in both the up and down directions. Therefore, unlike the case of hanging fluorescent linear lamp fixtures which direct some light up as well as down, when light is projected onto the floor using prior art LED linear lamps no light gets projected onto the ceiling, and the portion of the ceiling above the light fixture can remain somewhat dark. This is referred to as the “cave effect”. Thus, if conventional LED linear lamps with their 180-270 degree of illumination are used, little or no light get projected onto the ceiling and this leads to the cave effect of dark ceiling with a lighted space below.
There accordingly remains a need for new designs of LED linear tube lamps that remain efficient and direct light where it is needed while not creating a cave effect.